The present invention relates to an apparatus for gathering information relating to status changes to be used in, for example, a supervisory control and data aquisition system (hereinbelow, abbreviated to "SCADA system").
In a SCADA system, data respectively gathered by a plurality of remote stations is supplied to a master station, and a supervisory control is performed on the basis of this data. Each of the remote stations includes a plurality of status-change detectors and a processor which gathers data from the detectors and transmits this data to the master station. Since the present invention is well-suited for application to such information gathering processing within the remote station, the processing within the remote station of the SCADA system will be explained below by way of example.
The status-change detector in the remote station has the functions of supervising one or more plant apparatuses and storing the occurrence of a status change in the plant apparatus. Usually, it includes one to two printed circuit boards. Methods in which data sensed by the status-change detector is supplied to the processor are broadly classified into two types. One of the methods is such that the processor calls the respective status-change detectors in succession, and the status-change detectors transmit the stored status-change data to the processor in response to the calls. The other method is such that the detector having detected the occurrence of a status change interrupts the processor, and the processor then reads the status-change data.
Since, in the former case, the status-change detectors are successively called, there is the problem that the read cycle of the status-change data becomes long when a large number of the status-change detectors are provided. As a consequence, a desired time resolution for a status change cannot be secured. More specifically, it is desirable to feed the status-change data into the processor within a time T.sub.o after the occurrence of the status change. Nevertheless, when the number of the detectors is large, the read cycle becomes greater than the time T.sub.o, and the desired time resolution T.sub.o cannot be ensured. In order to ensure the time resolution, therefore, an additional processor needs to be provided. In addition, since the processor calls the status-change detectors and performs predetermined processing irrespective of the presence or absence of the status change, it has a high overhead. This imposes restrictions on the periods of time of the other processing (the transmission of data to the master station, the control of plant apparatus, etc.) of the processor.
Since, in the latter case, the processor receives only the status changes, the overhead based on such operation can be made lower than in the former case. However, the processing of the interrupt of one status change takes a certain time T.sub.2. Therefore, when status changes are concentrated in one status-change detector, the time needed to process these interrupts (the number of the status changes times the processing time T.sub.2) can become longer than the desired time T.sub.o described above, and the time resolution cannot be ensured in this case either.